How can I get the number of rows and columns of a 2-D dynamic array in c++ -


i trying make functions(adding, subtracting, product, etc) taking arguments dynamic 2-d arrays.

for example, given 2 2-d double arrays each 1 having 2 rows , 2 columns:

double** m1;  m1 = new double *[2];  (int = 0; < 2; ++i) {     m1[i] = new double[2]; }  double k1 = 1.0; (int = 0; < 2; ++i) {     (int j = 0; j < 2; ++j) {         m1[i][j] = k1;         k1 += 2.0;     } }  double** m2;  m2 = new double *[2]; (int = 0; < 2; ++i) {     m2[i] = new double [2]; }  double k2 = 2.0; (int = 0; < 2; ++i) {     (int j = 0; j < 2; ++j) {         m2[i][j] = k2;         k2 += 2.0;     } }

and made adding function them. first checks whether 2 arrays have same numbers of rows , columns. arrays totally dynamic(both numbers of rows , columns) , declared pointers pointers, can't numbers of rows , columns.

for example, value of 

sizeof(m1[0])

is 4, is, bite size of double pointer (probably) but, value of 

sizeof(m1[0][0])

is 8 right size of double type.

i want adequate version of former.

double** add_m(double **m1_, double** m2_) { //here first part needs numbers of arguments' rows , columns.     if ((sizeof(*m1_) != sizeof(*m2_)) || (sizeof(**m1_) != sizeof(**m2_))) {         cout << "the sizes of matrices don't match each other";         return 0;     }     else { //here second part needs numbers of arguments' rows , columns.         int num_rows = sizeof(m1_[0])/sizeof(m1[0][0]);//??         int num_cols = sizeof(m1_[0][0])/sizeof(m1_[0][0]);//??         double** res;         res = new double*[num_rows];         (int = 0; < num_rows; ++i) {             res[i] = new double[num_cols];         }          (int = 0; < num_rows; ++i) {             (int j = 0; j < num_cols; ++j) {                 res[i][j] = m1_[i][j] + m2_[i][j];             }         }         return res;     } }

edit: of answers using existing tools vector, matrix. curiosity, no way of doing it? way making vector-like class or matrix-like class scratch?

it seems want handle matrices.

unfortunatelly, dynamic memory allocation doesn't store size of allocated memory.

these alternatives:

store matrix dimensions yourself

then need pass matrix dimensions parameter functions. example, this:

double** add_m(double **m1_, double** m2_, unsigned width, unsigned height)

use std::array or std::vector

these classes store number of elements.

#include <array> #include <vector> using namespace std; // ... array<array<double, 3>, 3> m1 = {{ {1, 0, 0}, {0, 1, 0}, {0, 0, 1} }}; unsigned height = m1.size(); unsigned width = m1[0].size(); vector<vector<double> > m2 = {{ {0, 1, 0}, {1, 0, 0}, {0, 0, 1} }}; height = m2.size(); width = m2[0].size();

make own matrix class

that keeps track of number of elements , encapsulates operations can done them.

use external library defines "matrix" class

if make quick google search find tone (keywords: linear algebra library).

these tested , liked:


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